A rolling bearing unit is used to rotatably support a wheel of a vehicle on a suspension. To control an Anti-lock Braking System (ABS) or a Traction Control System (TCS), there is a need to detect a rotational speed of the wheel. For this reason, rotatably supporting the wheel on the suspension and detecting the rotational speed of this wheel using a rolling bearing unit with a rotational speed detecting device in which the rotational speed detecting device is incorporated into the rolling bearing unit have recently been widely adopted.
As an example of a conventional structure of the rolling bearing unit with the rotational speed detecting device used for this purpose, a structure shown in FIGS. 12 and 13 is described in Patent Document 1. The rolling bearing unit 1 with the rotational speed detecting device having this conventional structure rotatably supports a hub 3 which is a rotatable ring at an inner diameter side of an outer ring 2 which is a stationary ring.
The outer ring 2 include double rows of outer ring raceways 4a, 4b on an inner circumferential surface thereof, and a stationary-side flange 5 on an outer circumferential surface thereof. The outer ring 2 is supported by a knuckle (not shown) constituting the suspension and is not rotatable when used.
The hub 3 is formed by combining a hub body 6 and an inner ring 7 and includes double rows of inner ring raceways 8a, 8b on an outer circumferential surface thereof The hub 3 is supported at the inner diameter side of the outer ring 2 in the same center as this outer ring 2. Specifically, the inner ring raceway 8a of an axially outboard row is directly formed at an axially middle portion of an outer circumferential surface of the hub body 6, and similarly the inner ring 7, on an outer circumferential surface of which the inner ring raceway 8b of an axially inboard row is formed is fitted around and fixed to a small diameter step part 9 formed at an axially inboard end side portion (where the axially inboard side refers to a center side of a vehicle body in a width direction in a state assembled to the suspension, whereas the axially outboard side refers to an outboard side of the vehicle body in the width direction. This applies to the entire specification and claims). An axially inboard end face of the inner ring 7 is pressed by a caulking part 10 formed by plastically deforming an axially inboard end of the hub body 6 radially outward. A rotation-side flange 11 for supporting the wheel at an axially outboard end of the hub body 6 is provided at a portion which protrudes axially outward with respect to an axially outboard end opening of the outer ring 2.
A plurality of rolling elements 12, 12 are provided between the outer ring raceways 4a, 4b and the inner ring raceways 8a, 8b, respectively, and the hub 3 is rotatably supported on the inner diameter side of the outer ring 2.
An encoder 13 is fitted around and fixed to a portion deviating from the axially inboard ring raceway 8b inward at an axially inboard end of an outer circumferential surface of the inner ring 7. This encoder 13 is formed by combining a support ring 14, which is formed of a magnetic metal plate in an approximate L-shaped cross section and is of an annular shape as a whole, and an encoder body 16 which is attached to a side surface of a circular ring part 15 constituting this support ring 14. Since this encoder body 16 is formed of a permanent magnet such as a rubber magnet into which ferrite powder is mixed and is of a circular ring shape as a whole, it is magnetized in the axial direction, and changes directions of magnetization in an alternate manner at regular intervals in a circumferential direction. Therefore, S and N poles are disposed on an axially inboard surface or a detected surface of the encoder body 16 in an alternate manner at regular intervals.
A seal ring 17 is installed between the axially outboard end opening of the outer ring 2 and the outer circumferential surface of the axially middle portion of the hub body 6, and a cap 19 is mounted on an axially inboard end opening of the outer ring 2. Thereby, opposite end openings of a space 18 in which the rolling elements 12, 12 and the encoder 13 are installed are sealed, and grease enclosed within this space 18 is prevented from leaking out to an external space or foreign materials which are present in the external space are prevented from invading this space 18.
The cap 19 includes a bottomed cylindrical cap body 20 made by injection-molding a synthetic resin and a fitting ring 21 which is formed in an L-shaped cross section by stamping a non-magnetic metal plate and is of an annular shape as a whole. The cap body 20 includes a cap cylindrical part 22 and a cap bottom part 23 which closes an axially inboard end opening of the cap cylindrical part 22. The fitting ring 21 is fixed (molded) to an inner diameter side portion of a tip of the cap cylindrical part 22. A mount part 24 expanded axially inward (increased in an axial thickness dimension) compared to the other portion is provided at a radially outer side portion of the cap bottom part 23. A portion of the mount part 24 which axially faces the detected surface of the encoder 13 (the encoder body 16) is formed with a through-hole 25 which axially penetrates there through. A bottomed cylindrical sensor insertion cup 26 made of a non-magnetic stainless steel plate is molded into the through-hole 25. This sensor insertion cup 26 is buried in the mount part 24 by insert-molding at the injection molding of the cap body 20. Further, a mounting nut 27, on an inner circumferential surface of which internal threads are formed, is also buried in a portion of the mount part 24 which deviates from the through-hole 25 by insert-molding.
A sensor unit 28 for detecting a rotational speed is supported and fixed to the cap 19. The sensor unit 28 includes a sensor 29 and a sensor holder 30. Since a magnetism detecting element such as a Hall element, a magnetic resistance element, or the like is provided for a detector, the sensor 29 changes an output signal in response to a change in characteristics of the detected surface of the encoder 13. The sensor holder 30 is formed by injection-molding a synthetic resin and includes an insertion part 31 for holding the sensor 29 and a mounting flange part 32 for fixing the cap 19. This sensor unit 28 is fixed to the cap 19 (the mount part 24) by screwing an external thread part of a bolt 34, which is inserted into a through-hole formed in the mounting flange part 32, onto an internal thread part of the mounting nut 27 in a state where the insertion part 31 is inserted to the sensor insertion cup 26.
According to the rolling bearing unit 1 with the rotational speed detecting device of the conventional structure having the configuration as described above, the wheel fixed to the hub 3 can be rotatably supported with respect to the suspension supporting the outer ring 2. When the encoder 13 is rotated together with the hub 3 with the rotation of the wheel, the N and S poles which are present on the detected surface of the encoder 13 alternately pass through the vicinity of a detection part of the sensor 29 facing the detected surface of the encoder 13 via a bottom plate part 35 of the sensor insertion cup 26. As a result, a direction of magnetic flux flowing in the magnetism detecting element constituting the sensor 29 is alternately changed, and a characteristic of this magnetism detecting element is alternately changed. In this way, a frequency at which the characteristic of the magnetism detecting element is changed is proportional to the rotational speed of the hub 3. Thus, when the output signal of the sensor 29 is sent to a controller (not shown), the ABS or the TCS can be suitably controlled. In the conventional structure, even in a state before the sensor unit 28 is assembled on an assembly process of, for instance, a vehicle builder, since the space 18 in which the encoder 13 is installed can be sealed by the cap 19 (and the sensor insertion cup 26), it is possible to effectively prevent foreign materials from sticking to this encoder 13.
However, in the conventional structure as described above, there is a possibility of causing the following problems.
That is, in the conventional structure, a set of upper and lower molding dies 36, 37 as shown in, for instance, FIG. 14 is used to manufacture the cap 19. Specifically, a molten synthetic resin is fed into a cavity 38 which is defined with these upper and lower molding dies 36, 37 brought into axially contact with each other and has a shape matched with an outer surface shape of the cap 19. Particularly, in the conventional structure, the synthetic resin is fed with the sensor insertion cup 26 set in this cavity 38 (insert molding is performed). When this insert molding is performed, the bottom plate part 35 constituting this sensor insertion cup 26 is brought into contact with a part of the lower molding die 37 in order to regulate a position at which the sensor insertion cup 26 is installed, and similarly a part of the upper molding die 36 is butted against (bitten into) an axially inboard surface (curved surface) of a bent part 41 which is a connecting part between a cylindrical part 39 and a flange part 40 which constitute the sensor insertion cup 26.
When the insert molding is performed in the way as described above, the cylindrical part 39 of this sensor insertion cup 26 may be elastically deformed (expanded) radially outward based on a pressing force for the sensor insertion cup 26 of the upper molding die 36. Injection molding is performed in this state, and thereafter, when the cap 19 is extracted from the cavity 38 (when the pressing force caused by the upper molding die 36 is removed), since the cylindrical part 39 is elastically restored (reduced in its diameter), there is a possibility of a gap occurring at a coupling surface between the outer circumferential surface of the cylindrical part 39 and a portion of the synthetic resin which is present around this cylindrical part 39. When a foreign material such as water is invaded into this gap, there is a possibility that this foreign material further progresses axially outward to enter from an axially inboard end of the coupling surface to the space 18.
Incidentally, when the synthetic resin is cooled and solidified, it is contracted by a reduction in volume, which is generally known. For this reason, it is also thought that a portion, which is present around this cylindrical part 39, of a synthetic resin of which the cap body 20 is formed is contracted, and thereby the gap resulting from the above-described cause is extinguished and reduced. However, an inner diameter dimension of the through-hole 25 is typically about 10 mm, and an amount of contraction associated with the solidification is slight. For this reason, it is difficult to completely extinguish the gap.